The Swedish patent specification 8206393-4 (corresponds to European patent specification 0160774) describes a process for decreasing the contents of iron and manganese in ground water flowing through an aquifer, wherein a "curtain-type" zone for the oxidation and precipitation of iron and manganese is created by intermittently introducing water containing oxygen or oxygen-releasing substances through a number of injection wells arranged around one or more extraction wells for purified water. During each introduction of water containing oxygen or oxygen-releasing substances the water is fed to only some of the injection wells and simultaneously water is drawn from nearby, adjacent or intervening injection wells. The oxygen-containing water thus introduced creates a suitable environment for certain bacteria so that these, in combination with purely chemical processes, will bring about precipitation of the iron and manganese in the existing ground layers which will thus serve as a filter. The oxygen-containing water is added intermittently, for example, for a period of about 24 hours.
The Swedish patent specification 8400190-8 (corresponds to European patent specification 0154105) describes a process for decreasing the content of nitrate in ground water by reducing the nitrate to nitrogen in a reduction zone (denitrification zone) created between a number of injection wells arranged around one or more extraction wells for purified water. The reduction zone is formed in the same manner as the above described oxidation zone but water containing denitrification organisms and/or substrate therefore is fed to the injection wells instead of water containing oxygen or oxygen-releasing substances. Normally only substrate (oxygen-consuming substance) is added, such as sucrose, methanol, ethanol, an acetate or molasses.
For the creation of such oxidation and reduction zones an extensive pipe system is required, for example, for oxygenating water in an oxygenator station and pumping water to and from the injection wells. Moreover, the wells need to have a rather large diameter, of about 125 mm for accommodating an submersible pump.